This invention relates to a method of adjusting the tension applied to a sheet, more particularly a method of adjusting the tension applied to a sheet pulled out of a sheet roll in a stepwise manner according to the changing diameter of the sheet roll, and a device for carrying out the method.
One conventional drug packaging device includes a sheet feed unit rotatably supporting a rolled sheet of heat-fusible packaging paper, and a sealing device provided in a feed path through which the packaging sheet is unwound and fed. Upstream of the sealing device, the sheet is folded in half, drugs are supplied, and the sheet is heat-sealed in the width direction and along the edges by the sealing device to seal the drugs.
When the packaging sheet runs out, a new paper roll is set in the paper feed unit, and the new sheet is unwound and fed. When the sheet is fed, it has to be tensioned uniformly so that the sealing position will not deviate due to inaccurate folding of the sheet. But actually, the tension applied to the sheet changes gradually because the diameter of the paper roll changes gradually as the sheet is unwound.
Examined Japanese utility model publication discloses a sheet tension adjusting device which can apply uniform tension to the sheet even when the diameter of the sheet roll changes. The sheet tension adjusting device disclosed in this publication has a roll support cylinder on which is detachably mounted a sheet roll. A plurality of roll diameter detection sensors are provided at the side of the sheet roll. Signals from these detection sensors are used to control the electromagnetic force produced by an electromagnetic brake provided in the roll support cylinder so that the braking force decreases stepwise as the roll diameter decreases.
With this conventional sheet tension adjusting device, the sheet length of the sheet, which changes as the sheet is unwound, is detected stepwise by the roll diameter sensors arranged in the diametric direction of the roll. Thus, when the roll diameter decreases to a point where the detection sensor rank changes, the braking force rank of the electromagnetic brake will fluctuate up and down for every rotation due to deviation of the axis of the core shaft, the weight of the sheet, or winding strain.
If this happens, the sheet edges cannot be aligned accurately when the sheet is folded in half. Complete packaging is thus impossible. Since the braking force rank changes sharply, the sheet may suffer a laceration in the width direction.
Light reflecting type detection sensors have a problem in that they are more likely to malfunction. Packaging sheets used for a drug packaging device include semi-transparent, or transparent heat-fusible paper and many other kinds of paper. If the end position of such a sheet changes, light is reflected differently, making it impossible to detect the reflected light as a signal. This deteriorates detecting accuracy. Further, it tends to meander under the influence of humidity variations. Detection accuracy may deteriorate due to uneven end faces.
A thermal printer is usually provided upstream of the position where the sheet is folded in half for printing the packaging sheet. If the sheet vibrates, its printing dots may chip, or the durability of a remainder indicator lamp may deteriorate.
An object of this invention is to provide a method and a device of adjusting the tension in a sheet which is free of the problems of a conventional method of adjusting the tension in a sheet.